The present work is the first reasonably complete and large-scale survey of the speculations and achievements of the most eminent man of science of Elizabethan England. It is based on a study of all Dee's traceable printed works and surviving manuscripts. Though biography has been throughout only a secondary consideration, new materials discovered during the investigation - information hitherto unknown, neglected, or generally inaccessible - has usually been fairly fully incorporated, while an attempt has been made, for the first time, to provide full and detailed reference and documentation for the sources of all establishable facts concerning Dee cited here, and the bibliographies contain what it is believed is a fairly comprehensive catalogue of such works as make any significant mention of him. The general theme and purpose is to locate Dee within a sixteenth century current of scientifically orientated "neo-Platonism," the distinguishing characteristics of which are discussed in an introductory chapter (and which, it is argued, made important contributions to the development of scientific theory and practice), to exhibit him as a thoroughly representative though outstanding champion of such mathematical idealism in this age, and to reveal a unity in outlook, aims and methods informing the apparent wide variety of his multifarious endeavours, by tracing their organic connections with his central philosophical position. Major fields in which his attentions were especially engaged - mathematics, cabalism, astronomy, astrology, "alchemy," "natural magic," etc. - are considered in a framework of prevailing contemporary opinions and controversy, and Dee's particular theories and investigations regarding these, in relation to the fundamental metaphysical principles he embraced. The sources, specific contents, character and influence of his various writings are examined in detail, though a full treatment of the Preface Dee contributed to the English Euclid of 1570, and of the Euclid itself, in these respects, as well as an account of its considerable importance in the Renaissance of mathematical studies and also for more general scientific development, has had to be reserved as the subject of a second, subsequent study, owing to the unavoidable bulkiness of this initial survey.